problem: combating the urban heat island effect

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"Open spaces and undisturbed land have given way to buildings and roads. Sealed with concrete and asphalt, these surfaces no longer allow water to infiltrate the ground. Dark rooftops and pavement absorb and store energy from the sun during the day and reflect it at night."

REDEFINE THE ROOF

In the urban setting, where land is sold by the square foot (sqft) rather than by the acre, space is becoming a very valuable commodity. To deal with public goods, we cannot divest ourselves to private interests, as the tragedy of the commons has proven time and time again.

Rather, we should look to the root causes of the problem, in order to find practical solutions. 

One way is to re-explore the roof. Traditionally, the roof has been seen as a necessity, rather than as a tool. Arguably more than the wall, it is the most integral part of any building, as it provides shelter from weather and shade from sun.

Criteria (1) is that it continue to provide these functions, while (2) doing so in a way that mitigates the adverse effects of impervious land cover and the problems associated with thermal storage (heat island effect).

iNtensive (N): Up to or more than a foot of soil. Pro (+) : can sustain larger plant coverings,  greater capacity to store water, greater insulation properties. Con (-) : Heavy (from 80 to 150 pounds per square foot [lbs/sf]), requires a lot of maintenance.

eXtensive (X): Less than a foot of soil. Pro (+) : lighter at about 40 lbs/sf, less maintenance. Con (-) : less capacity to store and clean water, lesser insulation properties, can only sustain small vegetation.

Arguably, if you are already going to use the resources and know how to retrofit your roof, you might as well create type N, given that the building's structural specifications can hold the heavier load. 

The benefits, however minute, of vegetation vastly outweigh the costs of installation. This is especially true in an urban setting, where due to near 99% impervious land coverage, dealing with storm runoff has increasingly proven to be a challenge. Plants provide an adequate solution, in how they can metabolise the water into more plant growth.

Note: A green roof makes no sense in Kuwait or Las Vegas, because water is especially precious in the desert. Conservation and preservation of water as our most coveted resource should be the priority driving design in those regions. 

For example: the Southwestern United States gets an average rainfall of less then (< 5) five centimeters a year. Evapotranspirative potential (the sum of the water the sun can evaporate and that plants can metabolize) is between (5 < 2) two and five meters  per year. Water is precious, make no mistake about it.

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